1. Field of the Invention
The present invention relates to liquid crystal display (LCD) devices. More particularly, the present invention relates to a thin film transistor (TFT) array substrate for an LCD device and a simplified method of fabricating the same.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices express pictures by selectively altering light transmittance characteristics of liquid crystal material within an LCD panel having a plurality of pixels arranged in a matrix. Light transmittance characteristics of the liquid crystal material can be selectively altered by generating an electric field through the liquid crystal material (i.e., driving the liquid crystal material) between a pixel electrode and a common electrode formed on opposing substrates.
Accordingly, a typical LCD panel includes a lower array substrate (i.e., a thin film transistor (TFT) array substrate) joined to, and spaced apart from, an upper array substrate (i.e., a color filter array substrate) to form a cell gap between the lower and upper substrates. Spacers are distributed within the cell gap to uniformly maintain the distance between the TFT array and color filter array substrates and liquid crystal material is arranged within the cell gap containing the spacers.
The TFT array substrate typically includes a plurality of signal wirings, a plurality of TFTs, and an alignment film coated thereon to impart an alignment to molecules of the liquid crystal material. The color filter array substrate includes a color filter for selectively transmitting light having predetermined ranges of wavelengths, a black matrix for preventing a light from being transmitted in regions outside the pixels, and an alignment film coated thereon to impart an alignment to molecules of the liquid crystal material.
The process used to fabricate the TFT array substrate described above is complicated and relatively expensive because it involves a number of semiconductor processing techniques that require a plurality of mask processes. It is generally known that a single mask process requires many sub-processes such as thin film deposition, cleaning, photolithography, etching, photo-resist stripping, inspection, etc. To reduce the complexity and cost associated with fabricating TFT array substrates, procedures have been developed to minimize the number masking process required. Accordingly, a four-mask process has been developed that removes the necessity of a mask process from a standard five-mask process.
FIG. 1 illustrates a plan view of a TFT array substrate of an LCD device, fabricated using a related art four-mask process. FIG. 2 illustrates a sectional view of the TFT array substrate taken along the II-II′ line shown in FIG. 1.
Referring to FIGS. 1 and 2, the TFT array substrate includes gate lines 2 and data lines 4 formed so as to cross each other on a lower substrate 1 to define a plurality of pixel areas 5, a gate insulating film 12 between the gate and data lines 2 and 4, a TFT 30 provided each crossing of the gate and data lines 2 and 4, and a pixel electrode 22 provided at each pixel area. The TFT array substrate further includes a storage capacitor 40 provided at a region where a storage electrode 28 and the gate line 2 overlap, a gate pad 50 connected to each gate line 2, and a data pad 60 connected to each data line 4.
Each gate line 2 applies a gate signal to a gate electrode 6 of a corresponding TFT 30. Each data line 4 applies a pixel signal to a corresponding pixel electrode 22 via a drain electrode 10 of a corresponding TFT 30.
In response to a gate signal applied from a gate line 2, a TFT 30 charges and maintains a pixel signal, applied to a corresponding data line 4, in the pixel electrode 22. Accordingly, each TFT 30 includes a gate electrode 6 connected to a corresponding gate line 2, a source electrode 8 connected to a corresponding data line 4, and a drain electrode 10 connected to a corresponding pixel electrode 22.
Further, each TFT 30 includes an active layer 14 overlapping the gate electrode 6 and insulated therefrom by a gate insulating pattern 12. Accordingly, a channel is formed in a portion of the active layer 14 between the source and drain electrodes 8 and 10. An ohmic contact layer 16 is formed on the active layer 14 and ohmically contacts the data line 4, the source electrode 8, and the drain electrode 10 in addition to an overlaying lower data pad electrode 62 and storage electrode 28.
During operation, an electric field may be generated between the pixel electrode 22 and a common electrode supported by a color filter substrate (not shown). The liquid crystal material has a particular dielectric anisotropy. Therefore, in the presence of the electric field, molecules within the liquid crystal material rotate to align themselves vertically between the TFT and color filter array substrates. The magnitude of the applied electric field determines the extent of rotation of the liquid crystal molecules. Accordingly, various gray scale levels of light emitted by a light source (not shown) may be transmitted by a pixel area by varying the magnitude of the applied electric field.
Each storage capacitor 40 consists of the gate line 2 and the storage electrode 28 overlapping the gate line 2, wherein the two conductors are separated by the gate insulating film 12, the active layer 14, and the ohmic contact layer 16. The pixel electrode 22 is connected to the storage electrode 28 via a second contact hole 42 formed through a protective film 18 and is connected to the drain electrode 10 via a first contact hole 20. Constructed as described above, the storage capacitor 40 allows pixel signals charged at the pixel electrode 22 to be uniformly maintained until a next pixel signal is charged at the pixel electrode 22.
Each gate line 2 is connected to a gate driver (not shown) via a corresponding gate pad 50. Accordingly, the gate pad 50 consists of a lower gate pad electrode 52 and an upper gate pad electrode 54. The lower gate pad electrode 52 is an extension of gate line 2 and is connected to the upper gate pad electrode 54 via a third contact hole 56 formed through the gate insulating film 12 and the protective film 18.
Each data line 4 is connected to a data driver (not shown) via a corresponding data pad 60. Accordingly, the data pad 60 consists of a lower data pad electrode 62 and an upper data pad electrode 64. The lower data pad electrode 62 is an extension of the data line 4 and is connected to the upper data pad electrode 64 via a fourth contact hole 66 formed through the protective film 18.
Having described the TFT array substrate above, a method of fabricating the TFT array substrate according to the related art four-mask process will now be described in greater detail with reference to FIGS. 3A to 3D.
Referring to FIG. 3A, a gate metal pattern, including the gate line 2, the gate electrode 6, the lower gate pad electrode 52, is formed on the lower substrate 1 in a first mask process.
Specifically, a gate metal layer is formed over the entire surface of the lower substrate 1 in a deposition technique such as sputtering. The gate metal layer consists of a an aluminum group metal, etc. The gate metal layer is then patterned using photolithography and etching techniques in conjunction with an overlaying first mask pattern to provide the aforementioned gate metal pattern.
Referring next to FIG. 3B, a gate insulating film 12 is coated over the entire surface of the lower substrate 1 and on the gate metal pattern. In a second mask process, a semiconductor pattern and a data metal pattern are provided on the gate insulating film 12. The semiconductor pattern consists of the active layer 14 and the ohmic contact layer 16. The data metal pattern consists of the data line 4, the source electrode 8, the drain electrode 10, the lower data pad electrode 62, and the storage electrode 28.
Specifically, the gate insulating film 12, a first and a second semiconductor layer, and a data metal layer are sequentially formed over the surface of the lower substrate 1 and on the gate metal pattern by deposition techniques such as plasma enhanced chemical vapor deposition (PECVD) and sputtering. The gate insulating film 12 typically includes an inorganic insulating material such as silicon nitride (SiNx) or silicon oxide (SiOx). The active layer 14 is formed from the first semiconductor layer and typically includes undoped amorphous silicon. The ohmic contact layer 16 is formed from the second semiconductor layer and typically includes an n+ amorphous silicon layer. The data metal layer typically includes molybdenum (Mo), titanium (Ti), tantalum (Ta).
A photo-resist film is then formed over the data metal layer and is photolithographically patterned using a second mask pattern. Specifically, the second mask pattern is provided as a diffractive exposure mask having a diffractive exposure region corresponding to a channel region of a subsequently formed TFT. Upon exposure through the second mask pattern and development, a photo-resist pattern is created wherein a portion of the photo-resist film remaining in a region corresponding to the channel region has a lower height relative to portions of the photo-resist film remaining in regions outside the channel region.
Subsequently, the photo-resist pattern is used as a mask to pattern the data metal layer in a wet etching process and to form the aforementioned data metal pattern (i.e., the data line 4, the source electrode 8, the drain electrode 10, and the storage electrode 28), wherein the source and drain electrodes 8 and 10 are connected to each other in a region corresponding to the channel region. Next, the photo-resist pattern is used as a mask to sequentially pattern the first and second semiconductor layers in a dry etching process and form the active layer 14 and the ohmic contact layer 16.
After the active and ohmic contact layers 14 and 16 are formed, the portion of the photo-resist having the relatively lower height is removed from the region corresponding to the channel region in an ashing process. Upon performing the ashing process, the relatively thicker portions of the photo-resist in regions outside the channel region are thinned but, nevertheless, remain. Using the photo-resist pattern as a mask, the portion of the data metal layer and the ohmic contact layer 16 arranged in the channel region are then etched in a dry etching process. As a result, the active layer 14 within the channel region is exposed, the source electrode 10 is disconnected from the drain electrode 12, and the remaining photo-resist pattern is removed in a stripping process.
Referring next to FIG. 3C, the protective film 18 is coated over the entire surface of the lower substrate 1, on the gate insulting film 12, on the data metal pattern, and on the active layer 14. In a third mask process, the first to fourth contact holes 20, 42, 56, and 66, respectively, are formed through the protective film 18 and gate insulting film 12.
Specifically, the protective film 18 is formed over the surface of the lower substrate 1, and on the gate insulting film 12, the data metal pattern, and the active layer 14 by a deposition technique such as plasma enhanced chemical vapor deposition (PECVD). The protective film 18 typically includes an inorganic insulating material such as silicon nitride (SiNx) or silicon oxide (SiOx), or an organic material having a small dielectric constant such as an acrylic organic compound, BCB (benzocyclobutene) or PFCB (perfluorocyclobutane). The protective film 18 is then patterned via an overlaying third mask pattern using photolithography and etching processes to thereby define the first to fourth contact holes 20, 42, 56 and 66.
The first contact hole 20 is formed through the protective film 18 to expose the drain electrode 10, the second contact hole 24 is formed through the protective film 18 to expose the storage electrode 28, the third contact hole 56 is formed through the protective film 18 and the gate insulating film 12 to expose the lower gate pad electrode 52, and the fourth contact hole 66 is formed through the protective film 18 to expose the lower data pad electrode 62.
Referring next to FIG. 3D, a transparent conductive pattern including the pixel electrode 22, the upper gate pad electrode 54, and the upper data pad electrode 64 are formed on the protective film 18 in a fourth mask process.
Specifically, a transparent conductive material is coated over the entire surface of the protective film 18 and in the first to fourth contact holes 20, 42, 56, and 66 by a deposition technique such as sputtering. The transparent conductive material typically includes indium-tin-oxide (ITO), tin-oxide (TO), or indium-zinc-oxide (IZO). In a fourth mask process, the transparent conductive material is patterned using photolithographic and etching techniques to thereby form the aforementioned transparent conductive pattern (i.e., the pixel electrode 22, the upper gate pad electrode 54, and the upper data pad electrode 64).
Accordingly, the pixel electrode 22 is electrically connected to the drain electrode 10 via the first contact hole 20 while also being electrically connected to the storage electrode 28, via the second contact hole 42. The upper gate pad electrode 54 is electrically connected to the lower gate pad electrode 52 via the third contact hole 56 and the upper data pad electrode 64 is electrically connected to the lower data pad electrode 62 via the fourth contact hole 66.
While the TFT array substrate described above may be formed using a four-mask process that is advantageous over previously known five-mask processes, the four-mask process can still be undesirably complicated and, therefore, costly. Accordingly, it would be beneficial to fabricate a TFT array substrate according to a less complex, and therefore less costly, process.